Some current UTRAN (UMTS terrestrial radio access network) architectures include high speed links between an RNC (radio network controller) and an ATM (asynchronous transfer mode) ATM network, and then lower speed links between the ATM network and each base station (BTS). The interface between the RNC all the way to the base station, including the ATM switch, is referred to as the Iub. Various types of traffic that are supported include delay sensitive (DS) typically consisting of circuit switched services such as voice, NDS (non delay sensitive) that is less delay sensitive than DS but not entirely insensitive to delay and typically used in the packet switched domain, and HSDPA (high speed downlink packet access) that is typically not particularly sensitive to delay, also used in the packet switched domain. BTSs have a “receive window” for each traffic stream that defines a period within which each packet is expected. The window for DS traffic is smaller than that for NDS and HSDPA traffic. For example, the receive window for DS traffic might be 5 ms, the receive window for NDS traffic might be 50 ms, and there might be no window for HSDPA traffic.
Call admission control is used to limit the amount of new streams/services that are added to the network. This approach can be used to guarantee there is enough bandwidth for DS traffic that is deterministic in nature. However, existing systems do not provide for congestion management. Congestion can result due to the fact that the high speed link from the RNC to the ATM switch can handle more traffic than the links to the BTSs.
If the traffic is too early or too late, for certain traffic types, the BTS simply drops (i.e. discards) the traffic thereby wasting available bandwidth on the lower speed links. More generally, each RAB (Radio Access Bearer) type may behave differently in the presence of congestion. For DS RABs, no congestion is expected because typically call admission control is structured to ensure this. For NDS RABs, when there is congestion, transport frames are discarded at the base station due to missed receive windows and this results in wasted Iub bandwidth, increased RNC processing due to RLC retransmits etc. For HSDPA RABs, congestion results in increased delay and sporadic periods of inactivity/stalling due to queue overflow discards and subsequent RLC retransmits.